The switching system of the above mentioned type, especially destined to transmit television signals received in a number of channels on a multiplex line to subscriber posts, is known from the Dutch Patent Application No. 7803048 (corresponding to U.S. Pat. No. 4,135,157). In said known switching system a number of multiplexers are used on the one hand for sequentially scanning the voltage controlled oscillators corresponding to said mixing stages and on the other hand to receive selection signals from the subscriber posts. Through each of the subscriber terminals of this exchange a digital selection signal is received representative for the channel selected by the subscriber post concerned. On the basis of this digital selection signal an impulse series is generated in the exchange of which the frequency is representative for the desired nominal oscillator frequency of the mixing stage oscillator. Said impulse series is compared with an impuls series derived from the oscillator concerned and if a difference is detected the oscillator will be adjusted. This is done when a new channel is selected by the subscriber, in which case the related mixing stage oscillator has to be detuned over a relatively large frequency distance, as well as when still the same channel should be received and the oscillator is only drifting from the nominal frequency for instance under the influence of temperature variations.
All subscriber terminals are cyclically scanned and the signal received from each subscriber terminal is processed in the above described way. That implies that during each scanning selection information has to be received from the subscriber terminal, also in case the subscriber has not put in new selection information in which case in the exchange in fact the same selection information as in the preceding scanning is processed. The selection signals generated by a subscriber post must be stored in this known system in such a way that they are available during each scanning.
If between the generation of a selection signal in the subscriber post and the reception of the television signal corresponding with this selection signal a relatively long time period passes, for instance more than a few seconds, then the subscriber will find this very inconvenient. To avoid this inconvenience each subscriber has to be scanned for instance once per second to be able to react within a reasonable short time onto the selection of another channel by the subscriber. During the cyclical scanning of all the subscriber terminals connected to the exchange there is therefore only a relatively short time period available for such subscriber terminal in which time period eventually the oscillator has to be adjusted or, in case another channel is selected, has to be detuned over a smaller or larger distance. As is indicated in said Dutch Patent Application No. 7803048 (corresponding to U.S. Pat. No. 4,135,157) it is possible that, dependent on the frequency difference between the selected channel and the real oscillator frequency, more than one scanning is necessary before the oscillator will reach the correct frequency corresponding to the selected channel. In that case during each scanning the oscillator is detuned over a little distance in the desired direction. That means, however, that, in case each subscriber is for instance scanned once per second and for instance six scannings are necessary to detune the oscillator assigned to the subscriber concerned to the desired frequency, approximately six seconds will pass before the subscriber receives the desired channel as a result of his generated selection signal. Furthermore this subscriber will during this detuning period receive all kinds of undesired signals because the oscillator is each time detuned over a part of the detuning distance and stops thereafter on a momentaneous value, is detuned further and stops again, etc. Because this time delay is, dependent on the distance over which the oscillator has to be detuned, not equal in all cases, the subscriber not only will find this very annoying but is furthermore tending quickly to think that something is not functioning properly. If one likes to restrict this phenomenon in the known system then per scanning a certain minimum time period should be available to detune the oscillator, which time period should be selected so long that, in each case the desired tuning of the oscillator is reached within a very restricted number of scannings. Preferably the whole system should be dimensioned such, that the oscillator can be detuned to each desired frequency into one cycle. That means, however, that the number of subscribers, which can be connected to such a known exchange is rather restricted.
As long as no new selection signal is generated by a subscriber in fact during each scanning the same selection signal as during the preceding scanning is received from this subscriber post and the corresponding frequency signal is compared with the signal derived from the real oscillator frequency. If the oscillators are accommodated into an outdoor housing in the neighbourhood of the houses into which the corresponding subscriber posts are present, which is the case in the known system, then said oscillators have to function under very diverging temperatures. It is not imaginary that in the summer in the full sun the temperature inside the housing will climb until 60.degree. a 70.degree. C. whereas in winter one has to reckon with temperatures of -15.degree. C. and -20.degree. C., dependent onto the local climatological circumstances. Because of these temperature influences the frequencies generated by said oscillators under the same nominal control voltage can vary very strongly, which may lead to a strong disturbance or even the total disappearing of the selected channel. Because of this reason the frequencies generated by the oscillators in said known system are periodically compared with the nominal frequencies, also in case the selection signal is not changed in relation to the preceding scanning. If the generated oscillator frequency is drifting away then an adjustment is possible during each scanning.
As is already explained above a certain minimum time period should be maintained for scanning each subscriber's post to be able to detune the related oscillator within an acceptable time period if the selection signal is changed. Because on the other hand the oscillators will drift relatively slow under the influence of temperature and other influences and especially the drift between two scannings will be very small, only a relatively small time period is necessary to adjust an oscillator if necessary. Because during the major part of these scannings the selection signal will not be changed and therefore during most of the scannings the oscillator only has to be corrected eventually if the oscillator is drifted over a small distance, for which relatively little time is necessary, indeed significantly less time than the above mentioned minimum time period, during each scanning cycle in this known system relatively much time is lost. In fact this known exchange is based on a compromise between a long detuning period, a short correction period and a minimum number of subscriber posts connectable to the exchange.
A further disadvantage is that during each scanning a signal related to the oscillator frequency is compared with an impuls series which in one way or another is generated on the basis of the selection signal received from the momentaneous scanned subscriber. That implies that in said known system an impulse oscillator has to be used which, during the scanning of each next subscriber post should be able to generate another frequency. If only one variable impulse oscillator is used then one has to take into account the transition phenomena when switching to a next subscriber post, so that the time necessary for scanning a subscriber post is extended. If a number of fixed impulse generators are used, one for each possible to select channel, then this large number of oscillators itself is a disadvantage and for the more switching means have to be used to couple one of these oscillators with the frequency comparison means. In all cases these oscillators should be extremely stable and not or hardly temperature-sensitive, because otherwise the signal related to the mixing stage oscillator frequency is compared with an impuls series which is itself insecure or instable, so that the whole checking procedure of the mixing stage oscillators overshoots his mark. This requirement, however, leads to a significant increase in the cost for these oscillators.
Apparently the frequency comparison means in this known system are embodied as a forward/backward-counter, of which the forward input for instance receives the nominal impuls oscillator signal derived from the selection signal and of which the backward input received the signal derived from the mixing stage oscillator or vice versa. If the frequencies of the signals are exactly equal then the counter will receive the same number of forward counting pulses as backward counting pulses and will therefore theoretically at the end of the comparison period be in the starting position, for instance in the zero position. However, that is not always true. It has appeared that very easily a swinging around the zero position will develop. If the counter at the end of the comparison period for instance ends with a content of one, then a control signal will be generated to correct said mixing stage oscillator. Although this will be a very small correction signal, the result of this correction signal can very easily be that in the next scanning the counter not ends in the zero position but with a content of -1, which causes an oppositely directed correction signal, etc. The result thereof is that the mixing stage oscillator will be frequency modulated in a low frequency rhythm related to the scanning frequency. In case of television signals this causes a very annoying picture disturbance which is certainly unwanted in cable television systems.
There is a further way to influence the oscillator frequency in this known system. In the control line of the voltage memory assigned to each of said oscillators a switch is inserted which is closed as long as the subscriber selects channels which he is allowed to receive. If, however, the subscriber generates a selection signal for receiving a channel for which he has no admittance, then this will be detected in the exchange by means of a separate unit and said unit now delivers a switch control signal for opening the switch assigned to this subscriber. The result thereof will be on the one hand that the subscriber is not receiving the selected channel, for which he has no reception allowance, because the oscillator does not receive a control signal and is therefore not detuned to this "forbidden" channel. However, the oscillator stays in principle tuned to the channel, which was received before and this tuning will not be changed as long as the subscriber does not generate a new selection signal for a channel, which he is allowed to receive. On the other hand, however, the switch will stay open, so that the oscillator signal is able to drift and therefore the reception of the channel, which is still transmitted, will deteriorate. Because of the open switch this drift of the mixing stage oscillator is not corrected. If the subscriber after selecting the channel, the reception of which is not allowed, does not do anything at all, then even the situation will appear that the voltage of the voltage memory assigned to this oscillator will decrease very slowly, so that the oscillator very slowly is detuned and the subscriber therefore will receive slowly various channels during a shorter or longer time period, amongst other channels for the reception of which he has in fact no admittance. This very unwanted situation is only ended when the subscriber selects a new allowed channel.